Power tool

ABSTRACT

A power tool  1  is connected to a battery pack  5  including an overcurrent detector  533  which detects overcurrent of a battery module  51  and outputs a detection signal, and an overdischarge detector  532  which detects overdischarge of the battery module  51  and outputs a detection signal. The power tool  1  includes a motor  2  which is driven by electric power supplied from the battery pack  5 , a trigger switch  31  for setting the power supply from the battery pack  5  to the motor  2  in ON/OFF state, and an FET  410  which sets the power supply to the motor  2  in ON/OFF state on the basis of the detection signal from the overcurrent detector  533  or the overdischarge detector  532.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. patent applicationSer. No. 11/850,884, filed on Sep. 6, 2007, based on and claims thebenefit of priority from the prior Japanese Patent Application No.2006-243168, filed on Sep. 7, 2006; the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a power tool, and particularly to apower tool which prevents overcurrent and overdischarge of a batterypack.

BACKGROUND Description of the Related Art

Heretofore, there has been known a codeless power tool (hereinafterreferred to as a power tool) such as a driver or a circular saw whichcan operate by supply of electric power from a battery pack. Largeelectric power is necessary to operate such the power tool. Therefore,recently, in addition to a nickel-cadmium battery and a nickel-hydridebattery, a lithium-ion battery is used widely as a charging battery ofthe battery pack.

The lithium-ion battery has an advantage that large electric power canbe supplied and size/weight reduction of a tool can be realized. On theother, the lithium-ion battery must strictly cope with the ignition ofthe battery and the life lowering thereof due to overcharge, overcurrentand overdischarge. For example, in the lithium-ion battery, it isthought that: when the overdischarge is produced or the over currentflows in a load, a battery temperature increases thereby to causelowering of the battery life or to cause breakdown of a motor of a powertool; or when overcharge is produced, the ignition of the battery isproduced. For the above problems, there has been known a battery packwhich includes a braking unit that interrupts supply of the electricpower to the power tool (for example, refer to Japanese Patent No.3,222,951). Japanese Patent No. 3,222,951 discloses FET (Field-EffectTransistor) as a breaking unit.

SUMMARY

There is a battery pack in which plural FET's are provided as a brakingunit in order to prevent the overcurrent and the overdischarge. Herein,also in case that the current larger than permissible maximum current ofthe FET flows in the power tool, the power supply to the power tool canbe surely interrupted by the plural FET's. Further, there is also abattery pack which is not used exclusively for a specified power toolbut can be used for various kinds of power tools in common. Therefore,depending on the used power tool, the number of the FET's included inthe battery pack cannot make the interruption of the overcurrentpossible. Namely, in case that the number of the FET's included in thebattery pack is small, and the current flowing in the power toolconnected to the battery pack is large, the current flows in the FET'sexcessively, and the FET's break down. To the contrary, in case that thenumber of the FET's included in the battery pack is large, and thecurrent flowing in the power tool connected to the battery pack issmall, the FET's become useless.

Therefore, the invention has an object to provide a power tool whichincludes the necessary number of braking units which interrupts the flowof the current when the overcurrent or the overdischarge is produced.

In order to solve the above problems, according to a first aspect of theinvention, a power tool connected to a battery pack including a batteryand a battery state monitoring unit which detects overcurrent oroverdischarge of the battery and outputs a detection signal, includes: adrive part driven by electric power supplied from the battery; a firstswitch unit for setting arbitrarily power supply from the battery to thedrive part in ON/OFF state; and a second switch unit for setting thepower supply to the drive part in ON/OFF state on the basis of thedetection signal.

Further, according to a second aspect of the invention, the power toolincludes a retaining unit which retains, in case that the first switchunit is in ON state and the power supply to the drive part isinterrupted by the second switch unit, the interruption state of powersupply to the drive part by the second switch unit while the firstswitch unit is in the ON state.

According to a third aspect of the invention, the power tool includes afirst signal output unit which outputs a first signal for turning on thesecond switch unit, and a second signal output unit which outputs asecond signal for turning off the second switch unit. Herein, in casethat the battery and the drive part are connected in the state where thefirst switch unit is turned on, the second signal is input into thesecond switch unit.

According to a fourth aspect of the invention, in the power tool, thefirst signal output unit includes a resistor and a condenser, the secondsignal output unit includes at least a resistor and a condenser, andtime constant by the resistor and the condenser is larger in the secondsignal output unit than that in the first signal output unit.

According to a fifth aspect of the invention, the power tool furtherincludes an informative unit which informs a state of the second switchunit of a user.

According to a sixth aspect of the invention, a power tool connected toa battery pack including a battery includes a drive part driven byelectric power supplied from the battery; a first switch unit forsetting arbitrarily power supply from the battery to the drive part inON/OFF state; a second switch unit for setting the power supply to thedrive part in ON/OFF state on the basis of a detection signal accordingto a state of the battery pack; a first signal output unit which outputsa first detection signal for permitting the power supply from thebattery to the drive part to the second switch unit; and a second signaloutput unit which outputs a second detection signal for interrupting thepower supply from the battery to the drive part to the second switchunit. Herein, in case that the battery and the drive part are connectedin the state where the first switch unit is turned on, the seconddetection signal is input to the second switch unit.

According to the power tool in the first aspect of the invention, thesecond switch unit which sets the power supply to the drive part inON/OFF state on the basis of the detection signal is provided not forthe battery pack but for the power tool. When the power supply to thedrive part is set in ON state, the current flows into the second switchunit. However, incase of a power tool in which large current flows,plural second switch units must be provided in parallel to disperse thecurrent. According to the power tool in the first aspect of theinvention, since the second switch unit is provided for the power tool,the necessary number of second switch units may be providedcorrespondingly to the current which its power tool requires. Hereby, itcan be prevented that: Due to a small number of the second switch units,the current amount per a second switch unit becomes excessive, so thatthe second switch unit breaks down. Further, since it is not necessaryto provide the unnecessary number of second switch units, cost can bereduced.

Further, according to the power tool in the second aspect of theinvention, there is provided the retaining unit which retains, in casethat the power supply to the drive part is interrupted by the secondswitch unit in the state where the first switch unit is turned on, theinterruption state of power supply to the drive part by the secondswitch unit while the first switch unit is in the ON state. Therefore,it is possible to prevent the power supply and the power interruptionfrom being repeated in a short period. Namely, though the OFF state ofthe second switch unit solves the overcurrent state, and thereafter theON state of the second switch unit restarts the power supply, theovercurrent state is soon produced again, so that the second switch unitis turned off again. The repeat of this ON/OFF of the second switch canbe prevented by the retaining unit. Also, while the first switch unit isin the ON state, the interruption state is retained in case that thesecond switch unit is interrupted so as to present the power tool frombeing driven again when overcurrent is removed by taking the power toolaway from the object. Accordingly, safety improvement can be made.

According to the power tool in the third aspect of the invention, thereare the first signal output unit which outputs the first signal forturning on the second switch unit, and the second signal output unitwhich outputs the second signal for turning off the second switch unit.Herein, in case that the battery and the drive part are connected in thestate where the first switch unit is turned on, the second signal isinput into the second switch unit. Therefore, once the second switchunit is turned off, while the first switch unit is turned on, it ispossible to prevent the electric power from being supplied again to thedrive part.

According to the power tool in the fourth aspect of the invention, thefirst signal output unit includes the resistor and the condenser, thesecond signal output unit includes at least the resistor and thecondenser, and time constant by the resistor and the condenser is largerin the second signal output unit than that in the first signal outputunit. Therefore, once the second switch unit is turned off, while thefirst switch unit is turned on, it is possible to prevent the electricpower from being supplied again to the drive part by simpleconstruction.

According to the power tool in the fifth aspect of the invention, thereis the informative unit which informs the state of the second switchunit of the user. Therefore, the user can recognize that the secondswitch unit is turned off and the power tool cannot be operated.

According to the power tool in the sixth aspect of the invention, thereare provided the first signal output unit which outputs the firstdetection signal for permitting the power supply from the battery to thedrive part to the second switch unit, and the second signal output unitwhich outputs the second detection signal for interrupting the powersupply from the battery to the drive part to the second switch unit.Herein, in case that the battery and the drive part are connected in thestate where the first switch unit is turned on, the second detectionsignal is input to the second switch unit. Therefore, once the secondswitch unit is turned off, while the first switch unit is turned on, itis possible to prevent the electric power from being supplied again tothe drive part.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram showing the constitution of a power tool anda battery pack according to one embodiment of the invention;

FIGS. 2A, 2B, 2C are diagrams explaining the operation when overcurrentor overdischarge has been detected in the power tool and the batterypack in the embodiment; and

FIGS. 3A, 3B are diagrams explaining the operation when the battery packhas been connected to the power tool in a state where a trigger switchof the power tool in the embodiment is in ON state.

DESCRIPTION OF THE EMBODIMENTS

A power tool according to one embodiment of the invention will bedescribed with reference to FIGS. 1 to 3.

As shown in FIG. 1, a power tool 1 in the embodiment is used inconnection with a battery pack 5. The power tool 1 and the battery pack5 are connected removably through a positive terminal 54, a negativeterminal 55, and an overcurrent/overdischarge output terminal 56.

The power tool 1 includes a motor 2 driven by electric power suppliedfrom the battery pack 5, a switch unit 3 including a trigger switch 31which is manually switchable, and a controller 4 which controls rotationof the motor 2.

The battery pack 5 is connected to the power tool 1 in a state where thebattery pack 5 has been previously charged at the predetermined voltageand more, thereby to supply the predetermined voltage between thepositive terminal 54 and the negative terminal 55. When the triggerswitch 31 is closed, a closed circuit through the motor 2 is formedbetween the positive terminal 54 and the negative terminal 55. The motor2 receives the predetermined electric power thereby to be driven.

The battery pack 5 includes a battery module 51 in which plural batterycells 510 are connected in series, a resistor 52, for detecting acurrent flowing into the batter module 51, connected between thepositive terminal 54 and the battery module 51, and a battery protectionIC 53 which detects overdischarge, overcurrent, and overvoltage (orovercharge) of each battery cell 510, and outputs a signal according tothe detection result to the power tool 1 or a charger.

The resistor 52 and the battery module 51 are connected, between thepositive terminal 54 and the negative terminal 55, to each other inseries. The battery cell 510 constituting the battery module 51 is acharge battery such as a lithium-ion battery. The battery protection IC53, when it monitors the overdischarge and the overcurrent of eachbattery cell 510, and detects the overdischarage or the overcurrent inany battery cell 510, outputs a signal for interrupting the power supplyto the motor 2 through the overcurrent/overdischarge output terminal 56to the controller 4. Further, when the battery protection IC 53 detectsthat the battery cell 510 is in the overcharge state, that is, when thebattery protection IC 53 judges the battery cell 510 to be in theovercharge state in case that the voltage of the battery cell 510 comesto 4.35V or higher because rating of the lithium-ion battery is 3.6V perbattery cell and the maximum charging voltage is 4.2V, the batteryprotection IC 53 outputs a signal for stopping charging through anovercharge output terminal 57 to a charger. The “overcurrent” means astate where the current flowing into a load exceeds a predeterminedvalue, and the “overdischarge” means a state where the residual voltageof each battery cell becomes lower than a predetermined value. In theembodiment, the current value of the overcurrent is 70 A, and thevoltage value of the overdischarge of one battery cell 510 is 2V.

The battery protection IC 53 includes a battery cell voltage detector530, an overvoltage detector 531, an overdischarge detector 532, anovercurrent detector 533, and a switch 58. The battery cell voltagedetector 530 detects the voltage of each battery cell 510, and outputs adetection result to the overvoltage detector 531 and the overdischargedetector 532.

Into the overvoltage detector 531, the voltage of each battery cell 510is inputted from the battery cell voltage detector 530. In case that thevoltage of any battery cell 510 is a predetermined value or higher, theovervoltage detector 531 judges that the overvoltage is produced.

Into the overdischarge detector 532, the voltage of each battery cell510 is inputted from the battery cell voltage detector 530. In case thatthe voltage of any battery cell 510 is a predetermined value or lower,the overdischarge detector 532 judges that the overdischarge isproduced, and outputs a signal for closing (turning on) the switch 58.

The overcurrent detector 533 detects a value of the current flowing inthe resistor 52, judges, in case that the detected current is over thepredetermined value, that the overcurrent is produced, and outputs asignal for closing the switch 58.

When the switch 58 is closed by the signal from the overdischargedetector 532 or the overcurrent detector 533, theovercurrent/overdischarge output terminal 56 and a ground line areconnected. Accordingly, in this case, the battery protection IC 53outputs the voltage of zero volts (L₀ signal) to the controller 4 of thepower tool 1.

The motor 2 of the power tool 1 is connected through the switch unit 3and the controller 4 to the positive terminal 54 and the negativeterminal 55. The switch unit 3 is connected to the motor 2, and includesa trigger switch 31 and an invert switch 32. The trigger switch 31 isconnected to the motor 2 in series, and turned on/off by user'soperation. The invert switch 32 is a switch for inverting the polarityof the motor 2 connected to the positive terminal 54 and the negativeterminal 55 thereby to change the rotational direction of the motor 2.

The controller 4, upon reception of the signal for interrupting thepower supply from the battery protection IC 53, breaks the closedcircuit for power supply to the motor 2, and stops the drive. Thecontroller 4 includes a main current switch circuit 41, a main currentswitch-off retaining circuit 42 and a display part 43.

The main current switch circuit 41 is composed of a field-effecttransistor (FET) 410, a resistor 411, and a condenser 412. In the FET410, a drain is connected to the motor 2, a gate is connected to theovercurrent/overdischarge output terminal 56, and a source is connectedto the negative terminal 55. The resistor 411 is connected between thepositive terminal 54 and the gate of the FET 410. The condenser 412 isconnected between the gate and the source of FET 410. A contact amongthe gate of FET 410, the resistor 411 and the condenser 412 is taken asa contact A.

FET 410, while the power is normally supplied from the battery pack 5 tothe motor 2, is in ON-state. Namely, when the power tool 1 and thebattery pack 5 are connected, the battery voltage is applied through theresistor 411 to the contact A (gate of the FET 410), so that the FETenters the ON state. On the other hand, when the battery protection IC53 detects the overdischarge or the overcurrent, and the voltage of 0 V(L₀ signal) is inputted from the overcurrent/overdischarge outputterminal 56 to the gate of the FET 410, the FET 410 enters OFF state andbreaks the power supply to the motor 2.

The main current switch-off retaining circuit 42 is composed of an FET420, a resistor 421, a resistor 422, and a condenser 423. In the FET420, a drain is connected to a gate of the FET 410 and theovercurrent/overdischarge output terminal 56, and a source is connectedto the negative terminal 55. Further, a gate is connected through theresistor 421 to the motor 2 and the drain of the FET 410, and connectedthrough the resistor 422 and the condenser 433 which are connected toeach other in parallel to the negative terminal 55. When the voltage isproduced in a contact B on the gate side of the FET 420, the FET 420 isturned on, and the contact A connected to the drain of the FET 420 isconnected to the negative terminal 55 (ground line). Since the contact Ais connected to the gate of the FET 410, the gate of the FET 410 is alsoconnected to the negative terminal 55, so that the FET 410 is turnedoff.

The display part 43 is composed of a resistor 430 and a display (LED inthe embodiment) 431, and is connected between the drain and the sourceof the FET 410 in parallel. In case that the trigger switch 31 is in theOFF state, or in case that the FET 410 is turned on, the trigger switch31 is turned on, and the power is supplied to the motor 2, since apotential difference does not exist between both ends of the displaypart 43, the display 431 does not light up. On the other hand, in casethat the overdischarge or the overcurrent is detected and the FET 410enters the OFF state, the potential difference is produced between thedrain and the source. Namely, since the current flows through theresistor 430, the display 431 lights up and displays that theoverdischarge or the overcurrent is being detected. Hereby, the user canrecognize that the power tool 1 cannot be operated due to theoverdischarge or the overcurrent.

The operations of thus constructed power tool 1 and battery pack 5 willbe described. Referring first to FIGS. 1 and 2, the operation in casethat the trigger switch 31 has been turned on in a state where the powertool 1 and the battery pack 5 are connected will be described.

FIG. 2( a) is a diagram showing ON/OFF timing of the trigger switch 31.FIG. 2( b) is a time chart of a voltage VA, a voltage VB, and a voltageVC at a contact A, a contact B, and a contact C in FIG. 1, in which avertical axis indicates the voltage VA, the voltage VB, and the voltageVC, and a horizontal axis indicates time. FIG. 2( c) is a time chart ofcurrent detected by the overcurrent detector 533 of the battery pack 5,in which a vertical axis indicates current I, and a horizontal axisindicates time.

Time t0 is taken as arbitrary time when the electric power is normallysupplied from the battery pack 5 to the motor 2 after the trigger switch31 has been turned on. Here, the trigger switch 31 is in the ON statefrom the time t0 to the time t3, and is turned off at the time t3.Further, as shown in FIG. 2( c), though the current I is I1 at the timet0, it increases with the lapse of the time (overcurrent value I2 at thetime t1).

When the overcurrent detector 533 of the battery pack 5 detects that thecurrent flowing in the resistor 52 has come to 12 that is theovercurrent value at the time t1, it turns on the switch 58. By ON ofthe switch 58, the voltage of 0 volt (L₀ signal) is outputted from theovercurrent/overdischarge output terminal 56, and inputted into the gateof the FET 410. Hereby, as shown in 2(b), the voltage VA kept at aconstant value lowers from the time t1; and when the voltage VA becomeslower than ON-voltage V1, the FET 410 is turned off. Hereby, the powersupply to the motor 2 is broken.

When the FET 410 is in the OFF state, the current path changes to a paththrough the resistor 421, the resistor 422, and the condenser 423, thatis, a path through the main current switch-off retaining circuit 42,with the result that the voltage VC at the contact C increases.Simultaneously, the voltage VB also increases, and the FET 420 is turnedon at the time t2 that is over the ON-voltage V1. Then, the contact A,that is, the gate of the FET 410 is connected to the negative terminal55.

In case that the main current switch-off retaining circuit 42 is notprovided, OFF of the FET 410 solves the overcurrent state, andthereafter, the FET 410 is turned on again to start the power supply.However, in this case, the overcurrent state is soon produced again, andthe FET 410 is turned off again. Namely, ON and OFF of the FET 410 arerepeated. However, in the embodiment, the main current switch-offretaining circuit 42 is provided. Therefore, even in case that theovercurrent is solved and the switch 58 is turned off, since the FET 420of the main current switch-off retaining circuit 42 is in the ON-state,the voltage of 0 V is kept supplied to the gate of the FET 410, so thatthe FET 410 retains the OFF-state. Accordingly, the interruption of thepower supply to the motor 2 is kept.

When the trigger switch 31 is turned off at the time t3, the voltage VBlowers and thereafter becomes lower than the ON-voltage V1 at the timet4, so that the FET 420 is turned off. After the FET 420 has been turnedoff once, the battery voltage is applied through the resistor 411 to thegate of the FET 410 again. Namely, the voltage VA rises smoothly, andthereafter the FET 410 is turned on at the time t8 when the voltage VAexceeds the ON-voltage V1. When the trigger switch 31 is turned offunder this state, the power supply to the motor 2 is allowed. Also incase of the overdischarge, the similar operation is carried out.

As described above, according to the power tool 1 in the embodiment, TheFET 410 which sets the power supply to the motor 2 in ON/OFF state isprovided not for the battery pack 5 but for the power tool 1. When thepower supply to the motor 2 is in the ON state, the current flows intothe FET 410. In case of a power tool in which large current flows,plural FET's 410 must be provided in parallel to disperse the current,or a battery pack having large rating capacity must be selected inresponse to the usable power tool. However, according to the power tool1 in the embodiment, since the FET 410 is provided for the power tool 1,in response to the current which the power tool 1 requires, the use ofthe FET 410 may be selected or the necessary number of FET's 410 may beprovided. Hereby, it can be prevented that: due to a small number of theFET's, the current amount per FET 410 becomes excessive, so that the FET410 breaks down. Further, since it is not necessary to provide theunnecessary number of FET's, and further, since the FET in response tothe power tool 1 can be selected, cost can be reduced.

Further, there is provided the main current switch-off retaining unit 42which retains, in case that the power supply to the motor 2 isinterrupted by the FET 410 in the state where the trigger switch 31 isturned on, the interruption state of power supply to the motor 2 by FET410 while the trigger switch 31 is turned on. Therefore, it is possibleto prevent the power supply and the power interruption from beingrepeated in a short period. Namely, though OFF of the FET 410 solves theovercurrent state, and thereafter ON of the FET 410 restarts the powersupply, the overcurrent state is soon produced again, so that the FET410 is turned off again. The repeat of this ON/OFF of the FET 410 can beprevented. Also in case of the overdischarge, the similar operation iscarried out.

Next, with reference to FIGS. 1 and 3, the operation in case that thepower tool 1 is connected to the battery pack 5 charged at a givenvoltage in the state where the trigger switch 31 is in ON state will bedescribed.

FIG. 3( a) is a diagram showing ON/OFF timing of the trigger switch 31.FIG. 3( b) is a time chart of voltage VA and voltage VB at a contact Aand a contact B, in which a vertical axis indicates the voltage VA andthe voltage VB, and a horizontal axis indicates time.

At the time t0, the battery pack 5 is connected to the power tool 1 inthe state where the trigger switch 31 is turned on. Herein, till thetime t6, the trigger switch 31 is in the ON state; and at the time t6,the trigger switch 31 is turned off. As shown in FIG. 3( b), as soon asthe battery pack 5 is connected to the power tool 1, both the voltage VAand the voltage VB increase.

In the embodiment, time constant of the circuit composed of the resistor411 and the condenser 412, and time constant of the circuit composed ofthe resistor 421 and the condenser 423 are set so that the voltage VBrises earlier than the voltage VA. In the embodiment, the resistor 411,the condenser 412, the resistor 421, and the condenser 423 have aresistance value 1 MΩ, a capacitance 1 μF, a resistance value 1KΩ, and acapacitance 1 μF respectively. Accordingly, the voltage VB rises earlierthan the voltage VA, and exceeds the ON-voltage V1 at the time t5, andthe FET 420 is turned on. When the FET 420 is turned on, the voltage VAat the contact A becomes 0 volt. Therefore, the voltage of 0 V (Losignal) is input into the gate of the FET 410, and the FET 410 is notturned on. The OFF-state of the FET 410 is retained till the triggerswitch 31 is turned off.

When the trigger switch 31 is turned off at the time t6, since thecharge of the condenser 423 is discharged by the resistor 422, thevoltage VB lowers. When the voltage VB becomes lower than the ON-voltageV1, the FET 420 is turned off. By OFF of the FET 420, a closed circuitleading to the negative terminal 55 through the positive terminal 54,the resistor 411, and the condenser 412 is formed in the power tool 1,and the voltage VA increases.

Thus, according to the power tool 1 in the embodiment, in case that thepower tool 1 is connected to the battery pack 5 in the state where thetrigger switch 31 is in the ON state, it is possible to prevent that theelectric power is supplied to the motor 2 and the operation starts,whereby improvement of safety can be realized. In order to obtain suchthe effect, the overcurrent detector 533 and the overdischarge detector532 are not required, and the switch 58 for informing the overcurrent orthe overdischarge of the controller 4 is also not required.

In the embodiment, the motor 2 is a drive part of the invention, thetrigger switch 31 is a first switch unit of the invention, the FET 410is an interruption unit of the invention, the main current switch-offretaining circuit 42 is a retaining unit of the invention, the displaypart 43 is an informative unit of the invention, the battery protectionIC 53 is a battery state monitoring unit of the invention, the resistor411 and the condenser 412 constitute a first signal output unit of theinvention, and the FET 420, the resistor 421, and the condenser 423constitute a second signal output unit of the invention.

The power tool 1 according to the invention is not limited to the aboveembodiment, but various modifications and improvements are possiblewithin the scope of the accompanying claims.

1. A power tool comprising: a battery pack including at least onelithium-ion cell; a motor driven by electric power supplied from thebattery pack; a current path connecting the motor and the lithium-cell,said current path including a first switch and a first FET; a batteryprotection IC electrically connected to the current path, the batteryprotection IC outputting a first detection signal when the batteryprotection IC detects that the current in the current path exceeds apredetermined current value and a second detection signal when thebattery protection IC detects that a voltage of the lithium-ion cell islower than a predetermined voltage value; said first switch beingswitchable between an ON state and OFF state depending on useroperation; said first FET being switchable from its ON state to its OFFstate responsive to the first detection signal or the second detectionsignal, whereby the current flows in the current path, when the firstswitch and the first FET are both in their respective ON states; and asecond FET positioned outside of the current path, said second FET beingconnected to said first FET via a circuit, said second FET beingoperable, in response to an overcurrent or low voltage, to change itsswitching state, wherein when the current in the current path becomeslower than the predetermined current value, the first FET maintains itsOFF state when the first switch is in its ON state after the first FETchanges to its OFF state, and wherein when the voltage of thelithium-ion cell becomes higher than the predetermined voltage, thefirst FET maintains its OFF state when the first switch is in its ONstate after the first FET changes to its OFF state.
 2. The power toolaccording to claim 1, wherein the battery pack comprises a plurality oflithium-ion cells.
 3. The power tool according to claim 2, wherein thebattery protection IC comprises a plurality of terminals, each of saidterminals being connected to one of the plurality of lithium-ion cells,whereby the voltage of each of the plurality of lithium-ion cells may bemonitored.
 4. The power tool according to claim 3, wherein the batteryprotection IC outputs the second detection signal when the voltage ofone of the plurality of lithium-ion cells becomes lower than apredetermined voltage value.
 5. The power tool according to claim 1, thebattery pack further including: a detection signal terminal foroutputting the first detection signal and the second detection signal.